Because they're insecure and feel an emotional attachment to a character they like, so they have to give the illusion of superiority by lying out of their ass about feats.
-Comicvine is notorious for this, although I'm not going to name names.
-Outskirt Battledome aren't really liars, the problem is that they're just so ****ing stupid and delusional that they actually believe the things they say even when proven wrong.